Electrical anti-interference insulators



" Jly 13, 1955 w. HoPwooD 3,194,879

ELECTRIGAL ANTI-INTERFERENCE INSULATORS Filed March 12. 1964 2 Sheets-Sheet 1 M nvenlor Z B Wma/14% Mw July 13, 1955 wQHoPwooD 3,194,879

ELECTRICAL ANTI-INTERFERENCE INSULATORS Filed March l2, 1964 2 Sheets-Sheet 2 M MW WMM United States Patent O 3,194,879 ELECTRICAL ANTI-INTERFERENCE INSULATORS Walter Hopwoocl, Liverpool, England, assignor to Pilkington Brothers Limited, Liverpool, England, a corporation of Great Britain Filed Mar. 12, 1964, Ser. No. 351,312 Claims priority, application Great Britain, Mar. 1, 1961,

Claims. (Cl. 174-140) 'This invention relates to electrical insulators and in particular to electrical cap and pin insulators for use either alone or as a part lof an insulator stack in supporting power lines in power transmissionk and distribution systems and this application is a continuation-inpart of patent application Serial No, 173,613, filed February 13, 1962, and now abandoned.

In insulators of this kind electrical discharges are known to occur in use between parts of the insulator or insulator assembly in particular around the metal cap and pin electrodes which are cemented to the dielectric body of the insulator. These discharges can give rise to troublesome radio interference, and it has been proposed to reduce radio interference from electrical discharges in insulators by providing conducting components which short circuit the parts of the insulator between which dischargestake place.

It is a main object of the present invention to provide an electrical cap and pin insulator in which, without employing such additional short-circuiting components, electrical discharges producing radio interference are minimised or prevented.

This object is achieved according to the invention by specially shaping the dielectric body of the insulator in such a way that electric discharges in the space around the pin when it is cemented into the head of the dielectric body are minimised or prevented. From this aspect the invention provides an electrical cap and pin insulator comprising a dielectric body consisting of a hollow head, an integral skirtextending radially and downwardly from the head, and an annular rib extending downwardlyffrom the head under the skirt and having an inner surface of conical shape splayed outwardly and downwardlyk from the inner surface of the head, the region where the rib joins the head being formed as an annular groove joining the inner surface ofthe head to said conical inner surface of the rib, an electricallyconductive ycap cemented over the outer surface of the head, an electricallyconductive pin cemented inside the head, and conducting means electrically connecting the whole surface of said groove to the pin, the depth and shape of the groove and the conical shape of the inner surface of the rib being such that in use the electric field in the dielectric body is shaped by the groove and the rib in such a way as to produce a low electric stress in thespace between the inner surface of said rib and the pm.

Another object of the invention is to minimise or prevent electrical discharges which would otherwise occur between the lower edge 'of the cap and the upper surface of the skirt of the dielectric body.

A further object of the present invention is to provide a specially 'shaped dielectric body for an electrical inice sulator, for example a toughened glass body or a porcelain body of a cap and pin insulator.

in order that the invention may be more clearly understood an embodment thereof will now be described, by way of example, with reference to the accompanying drawings in which:

FIGURE l is a section on a diameter through a cap and pin insulator according to the invention, and

FIGURE 2 is a diagram showing the shape of the electric eld in part of the dielectric body of theinsulator of FIGURE l when in use.

Referring to FIGURE 1 of the drawings, an electrical cap and pin insulator for supporting a high-voltage power line, comprises a dielectric body consisting of a hollow head 1 and an integral skirt 2 extending radially and downwardly from the head 1. The insulator is one of a number of insulators which are connected together to form an insulator assembly or stack for` supporting a high-voltage power line. The dielectric body'may be a moulded and toughened glass body, or a moulded porcelain body.

The insulator is connected to identical adjacent insulators by means of cap and pin connectors which are electrically-conductive. A cap 3 has a specially shaped internal cavity 4 which tits over the head 1 of the dielectric body, and the cap is attached to the head by a joint 5 of electricallyeconductive cement.

The inner surface of the lower part 6 of the cap slopes inwardly so that when the cement 5 has set, any force on the cap tending to pull the cap 3 and the dielectric body apart causes the lower part 6 of the cap to tighten on to the head 1 by reason of the outwardly splayed shape of the outer surface of the head 1.

An electrically-conductive pin connector 7 is cemented inside the head by a joint 8 of electrically-conductive cement. The upper end 9 of the pin 7 is of conical shape and increases in diameter towards the top of the pin. The diameter of the upper part l@ of the inside surface of the head also increases towards the top of the head so that when the insulator is assembled in an insulator chain any pull on the pin 7 causes the cement 3 between the upper end 9 of the pin and the conically shaped sur face 1i) of the inside of the head, to bind on to the surface 10, so increasing the strength of the' joint between the pin and the head.

--The lower end of the pin 7 is shaped as a ball 11 which engages in a socket 12 in the top of the cap 3 of the next lower insulator in the chain. Normally the ball 11 would be held in the socket 12 of the next lower insulator cap by a spring clip (not shown) in well known manner.

The dielectric body also includes integral annular ribs 13, 14 and 15. The ribs 14 and 15 extend downwardly underneath the skirt 2, and the rib 13 extends downwardly iromthe bottom ofthe head 1 under the skirt 2. The inner surface 16 of the rib 13 is of conical shape splayed outwardly and downwardly from the lower part 17 of the 4inner surface of the head 1, which part 17 is of substantially cylindrical shape.

The region where the inner surface 16 of thefrib joins the inner surface 17 of the head is formed as an annular groove 18 of curved cross-section which extends'into the dielectric body in that region. As isr clearly seen in the drawing the groove 18 joins the vinner surface 17 of the enea-,are

3 head to the conically splayed inner surface 16 of the rib 13.

The whole surface of the groove 18 is provided with a coating 19 of an electrically conductive material, for example sprayed metal, and the cement i which holds the pin 7 in the hollow head iills the groove 1S and terminates at the lower edge of the groove so that the whole surface of the groove is electrically connected to the pin 7 by conducting means constituted by the coating 19 and the electrically conductive cement The function of the groove in producing a low electric stress in the space between the inner surface 16 of the rib 13 and the pin 7 will be described below.

The dielectric body is formed with a second annular groove 20 which is formed in the upper surface of the body in the region where the upper surface 21 of the skirt 2 joins the outer surface 22 of the head 1. The groove 26 dips into the dielectric body at the bottom of the outer surface 22 of the head and is of flattened U-shape in crosssection so that it can easily receive and accommodate the thickened lower edge 23 yof the lower part 6 of the cap 3.

The whole surface of the groove Ztl is provided with a coating 24 of electrically conductive material, for example sprayed metal, and thecement 5 which cements the cap 3 to the head 1 of the dielectric body fills the groove 2t? so that the lower edge 23 of the cap 3 is embedded in cement in the groove 20. This arrangement ensures that the whole surface of the groove Ztl is electrically connected to the cap 3 through the cement in the groove and the coating 2d on the inner surface of the groove.

In the embodiment illustrated by way of example the cement `fills the grooves 18 and 2t), but the cement need not till the grooves las long as it is in good electrical connection with the coatings 19 and 24 on the groove surface.

If desired the whole of the outer surface 22 of the head may be coated with electrically conductive material such as sprayed metal, as may the whole of the inner surface 1d, 17 Aof the head. This added coating has been found advantageous in ensuring a good electrically conductive bond between the cap, the pin and the dielectric body.

The shape of the grooved dielectric body affects the electric eld in the space 25 between the surface 16 of the rib 13 and the pin 7. The depth and shape of the groove 18 are such that in use the electric field which is generated in the dielectric body is shaped by the groove 18 and the conical inner surface 16 of the rib 13, in such a way as to produce a low electric stress in the space 25 thereby minimising or preventing electric discharges in that space.

The special shaping of the electric field by the groove 18 and the rib 13 is illustrated in FIGURE 2 which shows equipotential lines, indicated by dotted lines, defining the electric field as it exists lin the dielectric body and the air surrounding the dielectric body, when the insulator is in use.

FIGURE 2 shows in detail a part of the insulator including the dielectric body, the cap 3 and the pin 7.

The dielectric material of the dielectric body, for example glass or porcelain is of a high electrical permittivity as compared to that of air, and this leads to a large proportion of the electric stress in the region where the field emerges from the bottom of the head 1 of the insulator being developed in the air around the insulator rather than in the dielectric material. When the insulator is connected in a chain of insulators a potential difference exists between the cap 3 and the pin 7, and this results in the field concentration which is indicated in FIGURE 2 in the head 1 of the dielectric body. At the bottom of the head 1 this field seeks to emerge into the surrounding air and the groove 18 formed in the dielectric body is of such a size, depth and position that, in combination with the conically shaped -inner surface 16 of the rib 13, it shapes the electric eld so that there is no concentration i of electric stress in the region 25 between the inner surface 16 of the rib 13 and the surface of the pin 7, where electrical discharges would otherwise occur.

There is shown in FIGURE 2 how equipotential surfaces 26 and 27 are diverted back into the dielectric material by the groove 1S, and are further diverted away from the region 2S when they emerge from the dielectric material by being refracted through the outwardly splayed surface 16 of the rib 13 so that the region 25 between the rib 13 and the pin 7 is a region of relatively low electric stress. The shape of the rib 13 and the groove 18 are such that in practice under any potential difference which might exist between the cap 3 and the pin 7 the electric stress in the region 25 never reaches a value which would cause a breakdown in the air gap and consequential discharges between the pin 7 and the rib 13, which discharges would give rise to radio interference.

In the same way there is a region of low electric stress in the air adjoining the .second groove 20, that is the region where the lower margin of the cap 3 is adjacent the top surface 21 of the skirt 2. The shaped conducting surface of the groove 20 deflects the equipotential l-ines 28 and 29 where the field emerges through the upper surface 21 of the skirt, in such a way that there is minimal electrical stress between the lower part 6 of the cap 3 and the upper surface 21 of the skirt. As the lower edge 23 of the cap is embedded in the electrically conductive cement 5 which fills the groove 20 the whole surface of the groove 2@ will be at the same electrical potential as the bottom of the cap, or substantially so, so that any danger of electrical discharges occurring in this region around the lower edge of the cap is minimal. Further, the fact that the edge of the cap is embedded in the cement means that there are no sharp edges or corners of the cap exposed to the air, which otherwise could give rise to a high charge concentration. v

The cement referred to herein may be any material effecting a metal/dielectric or dielectric/dielectric joint, for example electrically conductive cementitious material.

Although the invention has been described with reference to a cap and pin insulator forming part of an insulator stack for supporting a high-voltage power line, it will be apparent that a single dielectric body as illustrated in the figure may be employed with suitable insulator parts, for supporting a low-voltage transmission line.

Further, in a multi-part insulator, that is an insulator assembly having a number of dielectric parts bonded to each other without intervening metal components, two adjacent dielectric bodies may be attached to each other by a layer of electrically conductive cement which forms a joint. For example, a number of dielectric bodies according to the invention each comprising a hollow head and a radially extending skirt may fit into each other, a joint between adjacent bodies being effected by electrically conductive cement and the inner and outer surface of each head being grooved adjacent the periphery of the cement joint.

I claim:

1. An electrical cap and pin insulator comprising a dielectric body consisting of a hollow head, an integral skirt extending radially and downwardly from the head, and an annular rib extending downwardly from the head under the skirt and having an inner surface of conical shape splayed outwardly and downwardly from the inner surface of the head, the inner surface of the body being formed with an annular groove which forms a radial depression around the inner surface of the body and joins the inner surface of the head t0 said conical inner surface of the rib, an electrically-conductive cap cemented over the outer surface of the head, an electrically-conductive pin cemented inside the head, and conducting means electrically connecting the whole surface of said groove to the pin, the depth and shape of the groove and the conical shape of the inner surface of the rib being such that in use the electric field in the dielectric body is shaped by the groove and the rib in such a way as to produce a low electric stress in the space between the inner surface of said rib and the pin, thereby minimising or preventing electric discharges in the space.

2. An insulator according to claim 1, wherein the dielectric body is formed with a second annular groove in its upper surface in the region where the upper surface of the skirt joins the outer surface of the head,`the lower edge of the cap is located in said second groove, and second conducting means electrically connects the whole surface of said second groove to the cap thereby minimising orpreventing electric discharges between the lower edge of the cap and the upper surface of the skirt.

3. An insulator according to claim 2, wherein the cap is cemented to the head by electrically-conductive cement which fills the second groove, the lower edge of the cap being embedded in the cement in the second groove.

4. For :an electrical insulator, a dielectric body consisting of a hollow head, an integral skirt extending radially and downwardly from the head, and an annular rib extending downwardly from the head under the skirt and having an inner surface of conical shape splayed outwardly and 'downwardly from the inner surface of the head, the inner surface of the body being formed with an annular groove which forms a radial depression around the inner surface of the body and joins the inner surface of the head to said conical inner surface of the rib, 'the depth and shape of the groove and the conical shape of the inner surface of the rib being such that in rib 5. A dielectric body dielectric body is formed wi in its upper surface in a second region where surface of the skirt joins the outer surface of the head, which second groove has a U-shaped cross-section of such a depth and shape that from the skirt in said second from the head so as to pro air adjoining said second groove.

References Cited by the Examiner UNITED STATES PATENTS 3/ 35 Lusignan 3 35 Lusignan 4/ 39 Sleeman 4/ 4 1 Burleson 3 42 Taylor FOREIGN PATENTS 5/21 Germany. 3/47 Great Britain.

LARAMIE E. ASKIN, Primary Examiner. JOHN P. WILDMAN, Examiner.

according to claim 4, wherein the th a second annular groove the upper the electric eld emerging region is deflected away duce low electric stress in the 

4. FOR AN ELECTRICAL INSULATOR, A DIELECTRIC BODY CONSISTING OF A HOLLOW HEAD, AN INTEGRAL SKIRT EXTENDING RADIALLY AND DOWNWARDLY FROM THE HEAD, AND AN ANNULAR RIB EXTENDING DOWNWARDLY FROM THE HEAD UNDER THE SKIRT AND HAVING AN INNER SURFACE OF CONICAL SHAPE SPLAYED OUTWARDLY AND DOWNWARDLY FROM THE INNER SURFACE OF THE HEAD, THE INNER SURFACE OF THE BODY BEING FORMED WITH AN ANNULAR GROOVE WHICH FORMS A RADIAL DEPRESSION AROUND THE INNER SURFACE OF THE BODY AND JOINS THE INNER SURFACE OF THE HEAD TO SAID CONICAL INNER SURFACE OF THE RIB, THE DEPTH AND SHAPE OF THE GROOVE AND THE CONICAL SHAPE OF THE INNER SURFACE OF THE RIB BEING SUCH THAT IN USE THE ELECTRIC FIELD IN THE DIELECTRIC BODY IS SHAPED BY THE GROOVE AND THE RIB IN SUCH A WAY AS TO PRODUCE A LOW ELECTRIC STRESS IN THE AIR ADJOINING THE INNER SURFACE OF THE RIB. 